1. Field of the Invention
The present invention is related to the field of oil well logging instruments. More specifically, the present invention is related to apparatus within well logging tools which generate and detect signals which are amplitude and phase modulated in response to properties of earth formations.
2. Description of the Related Art
Oil well logging instruments are used to evaluate properties of earth formations surrounding wellbores drilled into the earth. Other logging instruments evaluate properties of fluids within the wellbore itself. One general class of well logging instruments includes devices which generate oscillating electromagnetic test signals. The test signals are imparted to the earth formations surrounding the wellbore, or to the fluids in the wellbore. A sensor, positioned on the tool to be responsive to the property of the formation or of the fluid, generates a signal which can be detected by various circuits in the tool. The amplitude and the phase of the signal generated by the sensor, relative to the original test signal, are indicative of the property being evaluated. For example, induction conductivity tools measure electrical conductivity of the earth formations by passing an oscillating current through transmitter coils on the logging instrument (tool). Receiver coils, positioned at appropriate locations along the tool, generate electrical voltages in response to the magnitude of currents induced in the earth formation, which currents themselves are indicative of the formation conductivity. In induction tools the amplitude of the sensor signal which is in phase with the test signal provides desirable information.
Another type of logging tool is used to determine the dielectric constant of the fluid in the wellbore. An oscillating test signal is conducted through a sensor which forms a parallel plate capacitor. The sensor is configured so that some of the fluid present in the wellbore moves between the plates. The capacitance of the sensor depends on the dielectric constant of the fluid in between the plates. Amplitude and phase difference from the amplitude and phase of the oscillating test signal depends on the capacitance of the sensor. The values of amplitude and phase difference can be used to calculate values of sensor capacitance, and therefore to estimate fractional volumes of water in the wellbore.
Systems for measuring amplitude and phase of the signal generated by the sensor are known in the art and include analog systems. Analog systems are described, for example in "Communication Systems and Techniques", M. Schwartz, W. Bennett and S. Stein, McGraw-Hill, New York, 1966. A drawback to using analog systems is that they typically require large, bulky components, which can be difficult to design to fit inside a well logging tool. Analog systems are also subject to inaccuracy resulting from temperature-induced drift in the ratings of system components.
It is also known in the art to substitute digital signal processing systems for analog systems. Digital signal processing systems are disclosed, for example, in "Multirate Digital Signal Processing", R. Crochiere and L. Rabiner, Prentice-Hall, Englewood Cliffs, N.J., 1983. Digital systems known in the art are computationally intensive particularly because calculation of phase and amplitude requires calculation of sine and cosine values at the signal frequency. Digital calculation of sine and cosine typically are performed using infinite series, which can be time consuming even on fast digital signal processing (DSP) processors. Digital systems known in the art are also difficult to synchronize with the original oscillating test signal, which makes accurate measurement of true phase change difficult.
It is an object of the present invention to provide a system for measuring the amplitude and phase of a sensor signal, generated in response to a test signal imparted to a medium under investigation, which requires much less digital computation and is synchronized to the test signal so that accuracy of phase measurement is improved.